The thing is, if time actually passed they would have to constantly come up with new character designs so that they all looked their age (for example, a teenage Mac would look totally different to an 8-year-old Mac). However, I get the impression that imaginary friends don't age as such - they're just created as a certain age and stay like that forever.
There was an episode in Billy & Mandy where a recurring minor character aged over the years, yet the main characters never did - they actually made a joke of it. I guess that's one way to get around it
